1. Field of the Invention
The present invention relates to a method for producing a multilayer printed wiring board and more particularly to a method for producing a multilayer printed wiring board whose degree of lamination and wiring density can substantially be improved.
2. Description of the Prior Art
In the conventional printed wiring boards, a conductive layer has in general been formed on the inner walls of holes for packaging parts and through-holes (hereinafter referred to as "via holes") by plating the packaging holes and via holes.
In addition, the conventional multilayer printed wiring board has been designed such that embedded through-holes (hereinafter referred to as "inner via holes") are formed within a part of inner layers as the degree of lamination of the multilayer printed wiring boards has become increasingly high.
Incidentally, the development of, for instance, IC's and LSI's of high integration, packaging density and speeding up thereof have recently been advanced with great strides and correspondingly, it has strongly been desired for an increase in the wiring density of the printed wiring board for packaging these devices.
As a method for increasing the wiring density of the printed wiring board, Japanese Patent Public Disclosure No. Sho 63-137499 (Application No. Sho 61-284629) discloses a method of producing a multilayer printed wiring board. This method will be detailed below with reference to the accompanying drawings. FIGS. 1 to 8 are schematic diagrams for explaining the process for producing a multilayer printed wiring board according to the method of this prior art.
Referring now to FIGS. 1 to 8, completed multilayer printed wiring board of this type has a structure comprising six conductive layers as shown in FIG. 8 and can be produced in the following manner.
(1) Via holes 41 (FIG. 1B) are formed through a plate 10
(1) Via holes 41 (FIG. 1B) are formed through a plate 10 (FIG. 1A) whose upper and lower faces are laminated with copper foil layers 21 and 22 respectively (hereinafter referred to as "copper-laminated plate") and then conductive layers 61 and 62 are formed through plating as shown in FIG. 1C.
(2) Circuit patterns are formed on the conductive layer 62 and the copper foil layer 22 according to the known tenting method which makes use of a photosensitive dry film resist 7 as shown in FIGS. 2A and 2B.
(3) The via holes 41 are filled with an epoxy resin 8, followed by drying and hardening the epoxy resin to give a multilayer plate 11 for 1.multidot.2 layers (see FIG. 3).
(4) As shown in FIG. 4, a multilayer plate 13 for 5.multidot.6 layers is prepared according to the same processes used above. Separately, a multilayer plate 12 for 3.multidot.4 layers, on which only conductive patterns are formed, is likewise prepared. Thereafter, these two multilayer plates 12 and 13 and the multilayer plate 11 for the 1.multidot.2 layers previously prepared are assembled through prepreg layers 51 and the resulting assembly is integrally formed by applying heat and pressure to give a multilayered board 1 shown in FIG. 4.
(5) Through-holes 3 for inserting parts, for electrically connecting patterns of the inner layers or for electrically connecting the upper face to the lower face are formed in the multilayered board 1 by a drill (FIG. 5) and then the assembly is plated to form a conductive layer 60 on the upper and lower faces and the inner walls of the through-holes (see FIG. 6).
(6) Desired circuit patterns are formed through the conductive layers 60 and 61 and the copper foil layer 21 on the upper-most layer and through the conductive layers 60 and 66 and the copper foil layer 26 on the lower-most layer according to the tenting method which mekes use of a photosensitive dry film resist 7 as shown in FIGS. 7 and 8 to give a desired multilayer printed wiring board.
In the conventional multilayer printed wiring board discussed above, it is necessary to form electrical connection between the upper most layer and the inner layers of the multilayered board 1 or between the upper most layer and the lower most layer thereof. To this end, the through-holes 3 must be formed in the multilayered board 1 by a drill and then a conductive layer must be formed within the holes after the board having a multilayer structure is formed. For this reason, the circuit patterns for a third copper foil layer 23 and a fourth copper foil layer 24 in the inner multilayer plate 12 must be designed so that the patterns do not cross the portions at which the through-holes 3 are subsequently formed and accordingly, it is not always easy to provide a multilayer printed wiring board having high wiring density.